Do not reference HTTP_VERSION internally

HTTP_VERSION is supposed to be a client supplied header. This usage
inside Rack is conflating it with SERVER_PROTOCOL, which imo is instead
also conflating it with the client's HTTP version from the request line.

In any of these cases, HTTP_VERSION is set when an existing Version
header doesn't already exist. So it's possible to send a Version header
to conflict with the expected behaviors.

According to the CGI spec
(https://tools.ietf.org/html/draft-robinson-www-interface-00)

>  Environment variables with names beginning with "HTTP_" contain
   header data read from the client, if the protocol used was HTTP.

This is an anscillary issue with Rack, but will leave that open for
discussion since this behavior already exists.

lib/rack/chunked.rb

@@ -62,7 +62,7 @@ def initialize(app)
     # a version (nor response headers)
     def chunkable_version?(ver)
       case ver
-      when "HTTP/1.0", nil, "HTTP/0.9"
+      when 'HTTP/1.0', nil, 'HTTP/0.9'
         false
       else
         true
@@ -73,7 +73,7 @@ def call(env)
       status, headers, body = @app.call(env)
       headers = HeaderHash.new(headers)
 
-      if ! chunkable_version?(env[HTTP_VERSION]) ||
+      if ! chunkable_version?(env[SERVER_PROTOCOL]) ||
          STATUS_WITH_NO_ENTITY_BODY.key?(status.to_i) ||
          headers[CONTENT_LENGTH] ||
          headers[TRANSFER_ENCODING]

lib/rack/common_logger.rb

@@ -50,7 +50,7 @@ def log(env, status, header, began_at)
         env[REQUEST_METHOD],
         env[PATH_INFO],
         env[QUERY_STRING].empty? ? "" : "?#{env[QUERY_STRING]}",
-        env[HTTP_VERSION],
+        env[SERVER_PROTOCOL],
         status.to_s[0..3],
         length,
         Utils.clock_time - began_at ]

test/spec_chunked.rb

@@ -18,7 +18,7 @@ def chunked(app)
 
   before do
     @env = Rack::MockRequest.
-      env_for('/', 'HTTP_VERSION' => '1.1', 'REQUEST_METHOD' => 'GET')
+      env_for('/', 'SERVER_PROTOCOL' => 'HTTP/1.1', 'REQUEST_METHOD' => 'GET')
   end
 
   class TrailerBody
@@ -81,7 +81,7 @@ def trailers
 
   it 'not modify response when client is HTTP/1.0' do
     app = lambda { |env| [200, { "Content-Type" => "text/plain" }, ['Hello', ' ', 'World!']] }
-    @env['HTTP_VERSION'] = 'HTTP/1.0'
+    @env['SERVER_PROTOCOL'] = 'HTTP/1.0'
     status, headers, body = chunked(app).call(@env)
     status.must_equal 200
     headers.wont_include 'Transfer-Encoding'
@@ -97,10 +97,10 @@ def trailers
       body.join.must_equal 'Hello World!'
     end
 
-    @env.delete('HTTP_VERSION') # unicorn will do this on pre-HTTP/1.0 requests
+    @env.delete('SERVER_PROTOCOL') # unicorn will do this on pre-HTTP/1.0 requests
     check.call
 
-    @env['HTTP_VERSION'] = 'HTTP/0.9' # not sure if this happens in practice
+    @env['SERVER_PROTOCOL'] = 'HTTP/0.9' # not sure if this happens in practice
     check.call
   end